This invention relates to circuits for phase shifting, and the adjustment of such circuits.
There are numerous applications of circuits for phase shifting. One common application is in radio transceivers. Many radio architectures, especially in low or zero IF receivers or transmitters employ complex mixers. In a typical configuration the mixer is arranged to receive and mix two input signals which are at the same frequency (usually the local oscillator frequency) but in quadrature, i.e. 90xc2x0 out of phase. It is important that the signals are precisely in quadraturexe2x80x94otherwise spurious xe2x80x9cimagexe2x80x9d signals are generated. A 90xc2x0 phase shifting circuit could be used to generate one of the quadrature signals from the other. FIG. 1 shows an example of a possible circuit.
In the circuit of FIG. 1 the signal generated by the local oscillator is received at node 1 and buffered by unit gain amplifier 2. The signal then passes through two parallel arms 3, 4 to nodes 5, 6 respectively. Arm 3 contains resistor 7 and is tied to ground by capacitor 8. Arm 4 contains capacitor 9 and is tied to ground by resistor 10. The RC networks in arms 3 and 4 act as phase shifting circuits. The values of the R and C components 7 to 10 are selected so that arm 3 causes a xe2x88x9245xc2x0 phase shift of the signal from amplifier 2 and arm 4 causes a +45xc2x0 phase shift of the signal from amplifier 2. Thus the signals at nodes 5 and 6 are in quadrature. The signals from each node 5, 6 then pass to respective unit gain amplifiers 11, 12 and inverters 13, 14 to generate I (in-phase) and Q (quadrature) and {overscore (I)} (not in-phase) and {overscore (Q)} (not quadrature) signals respectively for use by the mixer(s) of the transceiver.
The characteristics of the phase shifting circuit are sensitively dependant on the values of the R and C components 7 to 10. Therefore, it is difficult to ensure that the phase shifting circuit is precise enough to avoid unacceptable image signals in the output from the mixer. If the capacitors 8 and 9 do not have identical values then the I and Q signals are unlikely to be orthogonal. Variations in the absolute values of the components 7 to 10 affects the amplitude of the output signals, and where two mixers are being used this amplitude difference can cause their gains to differ, which reduces the image rejection performance.
Another approach to generating the quadrature signals is to use a divide by four circuit. A signal is first generated at four times the local oscillator frequency. This is passed to the divide by four circuit and the quadrature outputs are taken from different points in the divide by four circuit. However, especially in RF applications it is often inconvenient or impossible to generate frequencies as high as four times the local oscillator frequency.
There is thus a need for an improved phase shifting circuit.
According to the present invention there is provided a phase shifting arrangement for generating a set of mutually orthogonal signals, comprising: a phase shifting unit for receiving an input signal and having a first phase shift circuit for generating a first output signal phase-shifted by a first amount with respect to the input signal, a second phase shift circuit for generating a second output signal phase-shifted by a second amount with respect to the input signal, and a third phase shift circuit for generating a third output signal phase-shifted by a third amount with respect to the input signal; the phase shift caused by each of the first, second and third phase shift circuits being adjustable in response to a feedback signal; and a phase shift adjustment circuit for generating the feedback signal and arranged to receive the first, second and third output signals and a fourth output signal of the same phase as the input signal and thereby generate an error signal dependant on the deviation of the first, second, third and fourth signals from mutual orthogonality; and a feedback circuit arranged to receive the error signal and form the feedback signal in dependence thereon in so as to cause the first, second and third phase shift circuits to, on receiving the feedback signal, generate the first, second and third signals in closer orthogonality to each other and to the input signal.
The first, second and third phase shift circuits are preferably, but not necessarily, of the same type. The first phase shift circuit is suitably arranged to receive the input signal and shift its phase to generate the first output signal. The second phase shift circuit is suitably arranged to receive the first output signal and shift its phase to generate the second output signal. The third phase shift circuit is suitably arranged to receive the second output signal and shift its phase to generate the third output signal. The phase shifts performed by the first, second and third phase shift circuits are preferably the same.
Preferably the phase shifting arrangement forms a feedback loop. Thus, the phase shifting unit and the phase shift adjustment circuit are preferably connected as a feedback loop. Most preferably the loop has a stable state when the phase shifts performed by the first, second and third phase shift circuits are 900xc2x0. Preferably each of the first, second and third phase shift circuits comprises a pair of series-coupled phase shifting arrangements capable of performing equal phase shifts. Preferably the phase shifting arrangements each perform a 45xc2x0 phase shift when the loop has a stable state.
The phase shift adjustment circuit preferably comprises a first complex mixer for receiving and mixing the first, second and third output signals and a signal of the same phase as the input signal to generate the error signal. The phase shift adjustment circuit most preferably also comprises a second complex mixer for receiving and mixing the first, second and third output signals and a signal of the same phase as the input signal, and a combining unit for combining the outputs of the first and second complex mixers to generate the error signal. The two complex mixers preferably have the same circuit arrangement, but receive the first, second, third and fourth signals at different inputs. At its inputs analogous to those at which the first complex mixer receives the first and third output signals the second complex mixer suitably receives the second and fourth output signals.
The phase shifting arrangement may be part of a radio transmitter and/or receiver, which may also comprise a local oscillator for generating the input signal. The input signal and the first, second and third signals may represent 1, Q, {overscore (I)} and {overscore (Q)} signals for use by the radio transmitter and/or receiver, suitably in modulation or demodulation of signals received by or to be transmitted by radio.